1. Field of the Invention
This invention relates generally to the field of filtering contaminants from molten, or liquid phase metals and particularly to a means for separating non-metallic inclusions and contaminants from molten metal as it is flowed.
2. Background
In the melting, refining and forming of metals, typically when molten metals are cast, it is desirable to separate exogenous intermetallic inclusions from the molten metal. Such inclusions result, in molten metals, from impurities included in the raw materials used to form the melt, from slag, dross and oxides which form on the surface of the melt, and from small fragments of the refractory materials that are used to form the chamber or vessel in which the molten metal melt is formed. Such inclusions, if not removed from the molten state of the metal, can result in weakened points and/or porosity in the final formed and solidified metal body which is the eventual downstream end product of the melting operation.
Typically, in a metal casting operation, the metal melt is formed in a furnace wherein the constituent components are added in the form of unmelted scrap and/or refined virgin metal, deoxidizing agents in various forms (both solid and gaseous or a combination of both) and alloying elements. Very light (less dense) solids and gases tend to migrate to the surface of the melt where they either effervesce or float in combination with partially and completely solidified oxides known variously as slag and dross. The higher density impurities in the melt tend to remain in some degree of suspension in the liquid phase of the metal, or melt, as the fluid flow convection currents are generated within that melt by the heating means applied by the melting furnace.
In the melting operation, the furnace acts as a vessel to hold the molten metal as it is being melted and, depending on the particular species of molten metal or alloy being formed, for a period of time following melting, to refine the molten metal by way of the gases and low density impurities migrating to the surface. The molten metal is then transferred, typically to another vessel, such as, for example, a ladle, to transport it to the forming means, such as, for example, a mold. Alternatively, the molten metal may be drawn directly from the furnace and flowed by gravity through a channeling means to a forming means, such as a continuous caster. A variety of other methods, used for removing molten metal from a furnace and conveying it to a forming means, are well known to those with skill in the art.
During this transportation or conveyance phase, wherein the molten metal is moved from the melting furnace to the forming means, it is desirable to ensure that the dross or slag, from the surface of the melt, does not become included in the formed metal and, also, that the higher density, exogenous intermetallic inclusions in the melt are not included in that formed metal.
One method that is used to prevent the inclusion of exogenous intermetallic substances, including slag or dross, in the formed metal body is to filter the molten metal as it flowed from the melting furnace to the forming means. A variety of means for accomplishing this filtration are well known to those with skill in the art. Recent examples of this can be found in U.S. Pat. Nos. 4,444,377; 4,426,287; 4,413,813; 4,384,888; 4,330,328; 4,330,327; 4,302,502; 4,298,187; 4,258,099; 4,179,102; 4,159,104, 4,081,371; 4,032,124 and 3,869,282. There are many additional recent references readily available that demonstrate methods of filtering molten metal. Also, there are many older references available, which date further back into history and which show apparatus and methods for filtering molten metals, such as, for example, U.S. Pat. No. 3,006,473.
In such systems, a filter medium or filter element is used. The basic material property that is necessary in a filter medium is that it be formed from a high temperature material which will withstand the elevated temperatures of molten metals and be stable in such an environment. That is to say that the material must not be subject to deterioration from melting, chemical reactions or erosion at such elevated temperatures. Also, the filter medium must maintain structural integrity at such elevated temperatures. And, of course, to act as a filter, the filter medium must be capable of either entrapping or preventing the flow of solids, liquids, and semi-liquids, all of which are non-metallic or intermetallic, either by chemically reacting the filter medium material with such inclusions and/or by mechanically preventing the flow thereof through the filter medium, while still permitting and facilitating the flow of the molten (liquid) metal therethrough. Further, such filter media are used in production facilities in association with unskilled or semi-skilled labor and heavy industrial machinery, equipment and tooling. Thus, such filter media should exhibit a high degree of structural integrity, at room temperature, such that rough handling will not be detrimental.
Many different designs of filter medium are known to those with skill in the field. Also well known are the uses of many different materials for, and many different methods of fabricating, or producing, porous bodies which can be used as filters. U.S. Pat. No. 3,796,657, for example, teaches the use of a fluidized and sintered aggregate of particles to form a porous chromatographic filter medium for separating gases from liquids and different liquids from each other. U.S. Pat. No. 4,430,294, as another example, teaches the formation of porous nickel bodies by using reducing gases and carbon powder to form interstices in nickel powder, during a rapid sintering process, U.S. Pat. No. 4,285,828, as yet another example, teaches forming a porous aluminum body by combining aluminum powder with an expanding agent, such as a fine salt, hot pressing the mixture and dissolving the expanding agent from the pores of the body. U.S. Pat. No. 4,391,918, as yet another example, shows the impregnation of an open celled organic foam with a slurry composed mostly of aluminum oxide plus sintering aids. The organic foam is then burned out as the slurry is sintered to form an aluminum oxide ceramic foam which can be used to filter molten metals.
Many older patents teach the bonding of crystalline ceramic material, such as silicon carbide or alumina, with a vitrified ceramic material such as glass. Such is taught, for example, by U.S. Pat. No. 2,007,053. Also, it is known to directly sinter particles of ceramic material into a porous body to form a filter medium. Such is taught, for example, by U.S. Pat. No. 2,021,520. Finally, it is known to mix a ceramic material such as aluminum oxide with a combustible material such as carbon and burn out the combustible material during sintering to produce a porous body. Such is shown, for example, in U.S. Pat. Nos. 2,360,929 and 2,752,258.
One of the problems that is inherent in many of the filter media which are useful for filtering molten metals is that it is difficult to render the pores or passageways throughout the filter media substantially open but also controlled in sizing such that the molten metal will freely flow through the filter media cross section at a controlled rate and so that all solid matter, of a calculated size range or larger, will be uniformly blocked from passage through the full cross section of thickness of that filter media.
Another problem that is inherent in many of the known filter media which are useful for filtering molten metals is that the surfaces of the pores or passageways through the filter media are not smooth, and thus, are susceptible to non-uniform build up of solid matter which tends to adhere more readily to the non-smooth surfaces adjacent to the entry side of the filter medium, thus not fully utilizing the thickness of that filter medium to fully trap such solids. Further, non-smooth surfaces tend to create turbulence in the flow of molten metal, thus inhibiting the smooth flow thereof. These phenomena shorten the useful life of the filter medium as the flow of molten metal therethrough decreases at a relatively greater rate than if the full thickness of the filter media were usable to trap the solid matter.
Another problem that is inherent in many of the known, filter media is that it is difficult to localize sizes of pores or passageways through the filter media to form pore size gradations either through the filter media or from one side to another across the face thereof, when desired. Such gradations are useful in specialized situations for preventing the passage of mixed solid materials of various types, and enhancing the separation of certain gases from the molten metal, as it flows through the filter media. Such gradations may also be used to selectively control flow rates in specialized circumstances.
Yet another problem that is inherent in many of the known filter medium is that they are too brittle or too friable, or both, at elevated temperatures as well as at room temperature. Thus structural failure of the known filter media has been a major problem related to the economics of using such for filtering molten metal. The strength is known to be diminished by the presence of sharp corners, non-continuous ceramic structure, and large pores in the load bearing sections of the ceramic material. For example, filamentary pores, left behind after the formation of reticulated foam filter media, exhibit such defects.
The present invention provides a filter medium, and a means for producing it, without filamentary pores, but with relatively uniform cell sizes, with passageways or pores therebetween, with relatively smooth surfaces on the cell walls and the walls of the passageways or pores therebetween, with the edges, or discrete transition areas, between cells and their interconnecting pores, being rounded off or smoothed, and a means for forming a filter medium with cell size gradations or localization of cell sizes or passageway (pore) sizes. Also, the present invention provides a filter medium with a high degree of structural integrity both at room temperatures and at the elevated temperatures associated with molten metals.